Two principal types of magnetic materials exist: materials that exhibit “soft” or “hard” (also known as “permanent”) magnetic characteristics. These materials are differentiated on the basis of their B-H hysteresis loops, when the curves are plotted as magnetic remanence (B) versus coercivity (H). In a standard accepted orientation, magnetic remanence B is plotted on the Y (ordinate) axis and coercivity H on the X (abscissa) axis. The area of this curve is called the BH product or the “energy” of the material and is expressed in units of energy.
Soft magnetic materials are characterized by low magnetic coercivities, typically below 12.5 Oe, and high magnetic permeabilities. Magnetic permeability is the magnetic induction response of a material to an applied magnetic or coercive field and is in general not a constant or linear with the applied magnetic field. Soft magnets are used in applications where high magnetic permeability is required to achieve high magnetic induction. Such materials have low coercivity, such that the magnetic induction can be readily removed or reversed when the applied field is changed in magnitude or direction. Soft magnetic materials are also termed “low energy” in that the BH product is low. Such materials are used in transformers, generators, and electrical motors, among other applications. The materials which comprise the soft magnetic materials family include iron and its various alloys for low frequency applications, and at higher frequencies ceramic oxides based on iron oxide with various modifying additives are used due to their lower electrical conductivity and therefore lower electrical losses at the higher applied frequencies.
“Hard” or permanent magnetic materials are those which have both high coercivities and high magnetic induction. Typical coercivities are greater than 125 Oe. Such materials retain their magnetization after removal of the applied magnetic field and large magnetic fields must be applied to reverse or remove the residual magnetization. These materials are termed high energy in that a plot of their BH properties is a very open hysteresis curve and this curve has a significant area and therefore a high BH energy product. Hard magnetic materials find many applications in technology where a strong and permanent magnetic field is required such as in magnets for electrical motors, electrical meters, loudspeakers, etc.
One type of hard magnet composition is the magnet made from materials within the Nd—Fe—B system, commonly termed “Neo” magnets. Additions of small amounts of other materials are made to tailor the magnetic properties of these materials to desired characteristics. Such magnets are fabricated into required forms for various applications, for example, melts using a variety of foundry techniques such as casting followed by machining. Powder metallurgical techniques are also used on the powder forms of the material such as sintering or hot pressing with and without a binder. Such techniques have advantages in that the magnetic properties of the finished article are not diluted by the presence of a non-magnetic binder. However, such forming processes are expensive and labor intensive in that complex dyes and molds need be used and post-forming machining is typically required. Complex shapes are difficult and expensive to make using these methods.
Polymer bonded forms of the Neo materials are also made using injection molding, compression molding, calendering, rolling, or other methods known in the art. In these techniques the Neo powder is mixed and dispersed into a polymeric medium and then formed using the various techniques into a near-net shape. Polymers used to make flexible bonded magnets include DuPont HYPALON® chlorosulfonated polyethylene, DuPont TYRIL® chlorinated polyethylene, nitrile rubber, vinyl and others. Polymers used for stiff and/or rigid bonded magnets include acrylics, nylon, polyphenylene sulfide, DuPont TEFLON®, thermoset epoxies and others.
A limitation of the technique for making polymer bonded magnets containing Neo powders is the amount of residual polymer that remains after processing. For instance, a magnet such as one that might be mounted on a refrigerator containing Ba/Sr ferrite powders has about 75% of its weight in magnetic powder, the remainder being polymer. The high amount of residual polymer dilutes the magnetic properties of the finished magnet such that the total magnetic flux and total magnetic strength of the piece is reduced significantly. In addition, the above processes have several limitations. One is the inability to produce thin, durable films of below approximately 10 mil manufactured thickness. Another is forming mechanically flexible and complex shapes without significantly wasting materials in a trimming or machining process.
The compositions of the present invention utilize thick film technology to solve the problems discussed herein. The composition upon processing yields a low cost, patternable, high energy magnetically permanent material.